This invention relates to a rotating anode x-ray source, commonly used in research to provide a continuous duty source of x-rays for studying the structure and composition of materials, and more particularly, this invention relates to a rotating anode x-ray source having a means to continuously remove impurities from the surface of the anode during operation.
In using x-rays to study materials, the differences in the absorption of different wavelength radiation by a specimen can provide valuable information about the structure of the material. In some x-ray absorption studies it is desirable to use a x-ray source having a continuous frequency band and constant amplitude, to monochromatize it, and to measure the absorption of discrete frequencies of x-rays by the material. In other studies of materials, it is desirable to provide discrete x-ray frequencies (a line spectrum) for measuring the diffraction by a specimen's crystal structure. In either case, the actual x-ray spectra emitted from a diode x-ray source will depend upon accelerating voltages between the anode and cathode and the constitutent materials of the anode including any impurities that may be transferred to the anode from the cathode.
In a rotating anode x-ray generator, electrons are ejected from a heated filament and are accelerated through a potential difference towards the anode, which they strike with high velocity. At an electron's point of impact upon the anode, x-rays are produced which radiate in many directions. Radiation produced by the electrons colliding with the anode will have a continuous frequency spectrum and superimposed on the spectrum will be a series of sharp intensity maxima at certain wavelengths. These maxima are known as characteristic lines and their wavelength and magnitude depend on the target material.
During normal use of the x-ray generator, some filament material is transferred to the anode from the filament, hence, characteristic lines will appear in the output radiation spectrum which are dependent upon the material used in the filament. For example, if the filament is made of tungsten, tungsten characteristic lines will appear in the output x-ray spectrum, which may not be desirable for the particular purpose of the instrument.
In applications as described above where the output x-ray spectrum should be free from undesirable frequencies, the deposition of impurities on the anode will degrade the quality of the output spectra emitted from the x-ray source.
In-situ methods of cleaning an x-ray anode have been used which utilize sputtering of the surface to remove impurities from the anode. For example, U.S. Pat. No. 3,334,228 issued to R. A. Mattson discloses an x-ray spectrometer having an x-ray source with a continuously cleaned x-ray target. The invention disclosed by Mattson utilized a rotating anode x-ray source with a source of ions propelled down a tube held in close proximity to the rotating anode. The ions propelled down the tube sputter impurities off the rotating anode, which impurities thereafter settle in the tube to be removed at a later time during a manual cleaning process.
The Mattson apparatus for continuously cleaning the rotating anode source uses a tube, contoured to conform to the circular periphery of the anode almost contiguous with the anode but with a small clearance gap. An ionized gas introduced into the tube sputters impurities from the anode surface. The gap between the tube and the rotating anode as disclosed in Mattson must be sufficiently small to sustain a difference in the magnitude of the vacuum between the interior of the tube and the exterior of the tube. The close fit required by Mattson is not easily attainable and increases the cost of such a continuously cleaned anode. The slow rotation (about one revolution per minute) of the anode which is specified by Mattson is not compatible with the high power (15 kilowatt) rapidly rotating (6000 revolutions per minute) anode x-ray generator which is in common use.
In addition, the sputtering gas introduced into the tube which is ionized to produce sputtering ions further "loads" the vacuum source required to maintain a low pressure within the tube. The higher pressure associated with the introduction of a gas may result in attenuation of the x-rays produced in the source and also prematurely ages the filament used therein.
Accordingly, an object of the invention is to provide a method of in-situ cleaning of an anode used in an x-ray source.
It is another object of the invention to present a rotating anode x-ray source wherein the x-ray source may be operated while being continuously cleaned without detrimental effect on the filament or other operational components.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combination particularly pointed out in the appended claims.